Which of these numbers is prime? ${4,\ 9,\ 37,\ 39,\ 63}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 4 are 1, 2, and 4. The factors of 9 are 1, 3, and 9. The factors of 37 are 1 and 37. The factors of 39 are 1, 3, 13, and 39. The factors of 63 are 1, 3, 7, 9, 21, and 63. Thus, 37 is a prime number.